为什么我们在应用程序中获得两次当前会话？

Question

最近我做了一些工作来检查在我们的应用程序中在哪里提供与会话相关的代码， 即.获取当前会话(sessionFactory.getCurrentSession());

我在应用程序中看到过这段代码两次，一次是在 HibernateSessionRequestFileter 类

package com.persistence;

import java.io.IOException;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.hibernate.SessionFactory;
import org.hibernate.StaleObjectStateException;
public class HibernateSessionRequestFilter implements Filter {

    private static Log log = LogFactory.getLog(HibernateSessionRequestFilter.class);

    private SessionFactory sf;

    public void doFilter(ServletRequest request,
                         ServletResponse response,
                         FilterChain chain)
            throws IOException, ServletException {

        try {
            log.debug("Starting a database transaction");
            sf.getCurrentSession().beginTransaction();

            // Call the next filter (continue request processing)
            chain.doFilter(request, response);

            // Commit and cleanup
            log.debug("Committing the database transaction");
            sf.getCurrentSession().getTransaction().commit();

        } catch (StaleObjectStateException staleEx) {
            log.error("This interceptor does not implement optimistic concurrency control!");
            log.error("Your application will not work until you add compensation actions!");
            // Rollback, close everything, possibly compensate for any permanent changes
            // during the conversation, and finally restart business conversation. Maybe
            // give the user of the application a chance to merge some of his work with
            // fresh data... what you do here depends on your applications design.
            throw staleEx;
        } catch (Throwable ex) {
            // Rollback only
            ex.printStackTrace();
            try {
                if (sf.getCurrentSession().getTransaction().isActive()) {
                    log.debug("Trying to rollback database transaction after exception");
                    //sf.getCurrentSession().getTransaction().rollback();
                }
            } catch (Throwable rbEx) {
                log.error("Could not rollback transaction after exception!", rbEx);
            }

            // Let others handle it... maybe another interceptor for exceptions?
            throw new ServletException(ex);
        }
    }

    public void init(FilterConfig filterConfig) throws ServletException {
        log.debug("Initializing filter...");
        log.debug("Obtaining SessionFactory from static HibernateUtil singleton");
        sf = HibernateUtil.getSessionFactory();
    }

    public void destroy() {}


}

GenericHibernateDAO 类中的另一个类如下所示，

protected Session getSession() {

        if (session == null) {
            session = HibernateUtil.getSessionFactory().getCurrentSession();
        } else if (!session.isConnected()) {
            session = HibernateUtil.getSessionFactory().getCurrentSession();
        }
        return session;
    }

任何人都可以帮助我理解，为什么我们必须在两个地方获取当前会话？ 当我们开始事务时，我们将获取 currentsession ，与我们持久化或从数据库获取对象时相同，我们再次获取 currentsession ， 为什么会这样呢？

Answer 1

这看起来像 OpenSessionInView 模式的一个版本，其中收到请求时打开 Hibernate 会话并在响应呈现后关闭。

在过滤器中打开会话并启动事务。

然后处理请求并在 Dao 中调用 getCurrentSession() 只获取当前打开的会话，它不会创建新会话。

道有它的作用。
然后过滤器提交事务并关闭会话。